High-speed data communications systems need to support the aggregate bandwidth requirements of current and future applications such as supercomputer interconnection, high-quality video conferencing, and multimedia traffic. It has long been clear that these bandwidth requirements can only be met by using optical transmission technologies. Many current approaches favor packet switching and ATM (asynchronous transfer mode) technology, due to their flexibility. The most promising candidate for the future hardware backbone for such networks is dense optical WDM (wavelength division multiplexing), a method of multiplexing a large number of optical data channels on a wavelength basis, i.e. each wavelength is regarded as a different channel, and is routed and manipulated separately from all other wavelengths.
Dense WDM needs advanced optoelectronic components and subsystems, capable of handling the extremely high aggregate bit rates and traffic levels demanded by modern optical data communications systems. One of the most critical components needed for implementation of WDM packet-switched systems is an ultra-fast tunable filter--a wavelength selective element in which the central wavelength of the selected bandpass can be tuned externally and dynamically at a very high rate.
Fast tunable filters are known and available commercially, but the tuning speed of all currently known types falls far short of the requirements of future and even of some current optical data transmission systems. The most common optical filters are based on classical interferometers, and include Fabry-Perot and Bragg filters. Such filters are tuned by mechanically moving the resonator structure, and the tuning speed is therefore comparatively slow--typically of the order of milliseconds, or, for the very fastest types, several tens of microseconds.
Another type of tunable filter is based on the Acousto-Optical effect. Such components depend on the interaction between an acoustic wave generated in the device, and the optical signal inputted to the component. Tunability is achieved by altering the frequency of the acoustic wave, which can be simply accomplished by altering the frequency of the electronic signal used to generate the acoustic wave. These filters are, however, polarization dependent, which causes many practical problems. Tuning speeds are reasonably high, of the order of microseconds.
Yet another tunable filter is based on a micromachined semiconductor structure, where the thickness of one of the parts of the structure is altered electrically. Here too, tuning speeds of the order of microseconds can be achieved.
The next generation packet-switched WDM networks are being designed for use with traffic throughputs of the order of Tbits/sec. Such systems therefore require switching and tuning speeds of the order of one nanosecond, and it is evident that even the fastest of the above mentioned filter technologies falls woefully short of these requirements, by about three orders of magnitude.